1. Field of the Invention
The present invention relates to a coordinate input apparatus that outputs coordinate positional information by detecting a variation in capacitance caused by a touch of the operator's finger and designating the position indicated by the finger. The invention also relates to a manufacturing method of the apparatus.
2. Description of the Related Art
As a typical input apparatus for inputting graphics and character information, a coordinate input apparatus, which is referred to as "a tablet", is becoming widespread. In this coordinate input apparatus of the type which inputs a desired item of information by selecting characters or graphics indicated on the display screen, the following types have been put into practical use: a resistive type that detects a change in resistance as a switch operation caused by touching characters or graphics on the display with a finger tip, and a capacitive type that detects a change in capacitance between electrodes caused by a touch of a finger tip.
As an example of known coordinate input apparatuses of the capacitive type, a dielectric sensor substrate formed in a rectangular shape is placed on a circuit substrate, and these substrates are electrically connected to each other via flexible wiring substrates. Formed on the obverse surface of the sensor substrate is an array of a plurality of X electrodes disposed at a predetermined interval and extending in the X direction, and formed on the reverse surface thereof is an array of a plurality of Y electrodes disposed at a predetermined interval and extending in the Y direction. The same end of each of the X electrodes is used as a connecting portion, and the same end of each of the Y electrodes also serves as a connecting portion, a connector being mounted on each connecting portion. A flexible wiring substrate is connected at both ends to each connector and the circuit substrate so that the X and Y electrode arrays formed on the sensor substrate can be electrically connected to the circuit substrate via this wiring substrate.
In the coordinate input apparatus constructed as described above, the operator's finger tip is pressed against a desired position of the sensor substrate so that electric lines of force to travel to the Y electrodes from the X electrodes are partially absorbed into the finger, thus causing a change in the electric lines of force. The position of the finger can thus be detected based on this change in capacitance.
As described above, in the conventional capacitive coordinate input apparatus, the X and Y electrodes mounted on the obverse and reverse surfaces of the sensor substrate are electrically connected to the circuit substrate via flexible wiring substrates. It is thus necessary that connectors to be coupled to the X and Y electrodes, respectively, be attached to the obverse and reverse surfaces of the substrate. This inevitably increases the thickness of the overall apparatus by an amount equal to the connectors. Moreover, a flexible wiring substrate coupled to each connector inevitably projects outwardly from the lateral side of the sensor substrate in order to be guided to the circuit substrate. This increases the external dimensions of the overall apparatus.
Additionally, in the conventional capacitive coordinate input apparatus, a protective sheet provided with an acrylic resin-formed adhesive sheet on its reverse surface is attached onto the sensor substrate. However, it is difficult for the protective sheet, formed of a sheet member as small as 0.2 mm in thickness, to adhere to match the undulating surface of the sensor substrate. Accordingly, it is likely, as illustrated in FIG. 13, that numerous irregularly-shaped bubbles B are disadvantageously produced on the sensor substrate 65 due to the thickness of an X electrode 64 (from 7 to 10 .mu.m) when the protective sheet 63 is attached onto the sensor substrate 65 via the adhesive layer 62. Air within the bubbles B trapped in the adhesive layer 62 produces an adverse influence on the capacitance. Thus, high precision cannot be easily achieved with the use of the conventional apparatus, for example, the linearity of the coordinate positions detected by the linear movement of a finger on the protective sheet 63 is significantly varied depending on which portion of the protective sheet 63 the finger contacts.
On the other hand, the protective sheet 63 with a reduced thickness is more likely to match the undulating surface of the sensor substrate 65. However, this only results in undulating the surface of the protective sheet 63 which serves as the operational surface. Consequently, the problem of variations in capacitance causing poor operational performance cannot be solved.